Why don't u try it out?? Use battery monitoring apps like Battery graph or batteryeye and see how the performance and battery life changes. Everyone uses their phone differently so it's best to do a test for yourself, it's not hard or anything, just let your phone run for a whole day doing what you normally do, and switch to a different overclock/underclock and try it out.
EDIT: In fact i'll help u out even more, read these 2 posts, that's all you need on overclocking. http://talk.maemo.org/showpost.php?p...&postcount=774 http://talk.maemo.org/showpost.php?p...postcount=3947